Activation of the central pathway of the baroreceptor reflex, a possible mechanism of the hypotensive action of clonidine

Summary

Arterial blood pressure, heart rate and discharges in the preganglionic splanchnic and a postganglionic renal sympathetic nerve were recorded in cats anaesthetized with urethane. Electrical stimulation of the posterior hypothalamus or the fastigial nucleus of the cerebellum elicited an immediate increase in sympathetic nerve activity and a rise in blood pressure and heart rate. The stimulation-induced discharge pattern in the sympathetic nerves was characterized by a strong initial burst followed by a phase of inhibition and a final stabilization of the discharges at a level definitely lower than the initial burst. This pattern was reversibly converted into a constant high amplitude firing during a lowering of the blood pressure by bleeding the cats and irreversibly so after cutting the buffer nerves. These findings indicate that the inhibitory phase of the sympathetic discharge pattern during central stimulation is due to the rise in blood pressure and the ensuing baroreceptor-reflex activation.

Clonidine (0.03 and 0.1 mg/kg i.v.) reduced the spontaneous sympathetic nerve activity and lowered blood pressure and heart rate. The sympathetic discharges evoked by central stimulation were partially inhibited by clonidine, an effect which could be overcome by raising the voltage used for central stimulation. Independent of the strength of this stimulation an inhibitory phase in the evoked discharge pattern was not observed after clonidine, even when the low blood pressure due to the drug action was raised by a noradrenaline infusion. After clonidine, the evoked discharge pattern closely resembled that after simultaneous stimulation of both sinus nerves and the hypothalamus or the fastigial nucleus, and it was not altered by additional stimulation of the sinus nerves. These observations have led to the hypothesis that clonidine causes a long-lasting activation within an as yet unidentified part of the central pathway of the depressor baroreceptor reflex. In view of the well-known α-adrenoceptor stimulating property of clonidine, and since the central effect of clonidine was antagonized by the α-adrenoceptor blocking agent piperoxan, it is likely that the central part of the baroreceptor-reflex pathway is modified by or contains adrenergic neurones.